Cement shapings include cement, mortar and concrete products used in civil engineering and construction fields such as asbestos cement plates, pulp cement plates, wood wool cement plates, wood chip cement plates, GRC (glass fiber-reinforced cements), cement roof tiles, mortar plates, terrazzo blocks, concrete plates, concrete piles, Hume concrete pipes, U-shaped grooves, concrete blocks, tetrapods, concrete ties and ALC (Autoclaved Light-weight Concrete).
Although varying depending on product types, the conventional methods for producing concrete shapings generally comprise compounding Portland cement, water and aggregates, kneading these materials, filling the mixture into a framework, pre-curing the mixture at ordinary temperatures (ambient temperature) for 2 to 4 hours, elevating the temperature of the pre-cured product to 60.degree. to 70.degree. C. using steam at a rate of 11.degree. to 33.degree. C. /hr, maintaining the curing temperature between 60.degree. and 70.degree. C. for 4 to 8 hours, stopping the supply of steam, and allowing the cured product to cool under natural conditions for 6 to 12 hours before removing the framework. (see Unexamined Published Japanese Patent Application Nos. 20252/80, 22151/82, Japanese Patent Publication No. 17871/82, and Technical Bulletin No. L-001, "Steam Curing-Report of ACI 517 Committee", published by Nisso Master Builders Co., Ltd., August, 1965).
The productivity of the conventional process is 1 to 3 cycles per day, which means 1 to 3 lots of cement shapings can be produced a day. If the production time is simply shortened to increase the process productivity, thoroughly hardened concrete products are not obtained. If the pre-curing time is shortened while the temperature elevation rate and curing temperature are increased, the structural integrity of cement shapings is impaired and products with such defects as expansion, blistering or cracking will result.
A method has been proposed to shorten the shaping cycle of the conventional steam curing process as described in Unexamined Published Japanese Patent Application No. 110420/75. This method comprises providing a hydraulic cement composition by compounding normal Portland cement with at least 5 wt% of early strength cement (i.e., a mixture of calcium aluminate, lime and gypsum), and optional components such as sodium carbonate and organic acids (e.g., gluconic acid, citric acid, tartaric acid, and Na or K salts thereof), and an aluminum powder (as a foaming agent), water and fine aggregates; placing the composition in a framework; precuring the composition for 1 to 2 hours; steam curing the pre-cured product at high temperature and pressure (180.degree. C. and 10 atmospheric pressure, with the temperature elevated at a rate of 80.degree. C. /hr) or steam curing the same at atmospheric pressure (60.degree. C. with the temperature elevated at a rate of 40.degree. C. /hr); cooling the cured product to room temperature over a period of 4 to 5 hours; and disassembling the framework to recover a lightweight shaped cement product having a compressive strength of 82 to 90 kg/cm.sup.2.
This method aims at shortening the shaping cycle by increasing the rate of temperature elevation after placing the hydraulic cement composition in a framework, but the result thereof is far from being satisfactory since considerable pre-curing time is necessary and as many as 5 to 6 hours are required until the steam-cured product cools down sufficiently to permit the disassembling of the framework.
The present inventors previously made various studies on cement admixtures to check whether the precuring step and the cooling step subsequent to steam curing would be necessary at all. As a result, the inventors obtained the following observations: if a specific sulfate is used as a cement additive, the cement composition shaped in an open framework need not be pre-cured and can be immediately cured at a temperature of 80.degree. C. or higher by employing a temperature elevation rate of 40.degree. C. /hr upward; this rapid and high temperature curing produces a sound and immaculate cement shaping in 1 to 3 hours; and the productivity of this method is at least 10 cycles/day. On the basis of these findings, the present inventors filed Japanese Patent Application No. 125571/83 on a process for producing a cement shaping comprising placing in a framework a hydraulic cement composition comprising a hydraulic cement and 0.1 to 20 wt% of at least one additive selected from the group consisting of sulfate salts of lithium, aluminum, potassium and thallium, as well as sulfate complex salts containing these metals, and curing the shaped cement composition at elevated temperatures.